The invention relates to a stereo receiver circuit for progressing a multiplex signal comprising a pilot signal by means of a pilot signal detector which detects frequency components in the multiplex signal at the frequency of the pilot signal and which controls a Schmitt trigger for influencing the mono/stereo operation, said circuit also comprising a control circuit capturing the noise components in the multiplex signal and acting on the Schmitt trigger.
Such a circuit, which is essentially known from DE-AS No. 27 53 199 may be used in an FM stereo radio receiver but also in a television receiver which can receive stereophonic audio signals, for example, in accordance with the U.S. standard.
At small reception field strengths the output signal of the pilot signal detector is subject to fluctuations due to the noise which is superimposed on the multiplex signal, which fluctuations may lead to a constant switching between mono and stereo operation and to flickering of a stereo indicator coupled to the output of the Schmitt trigger. To avoid these disturbing effects, a control circuit is arranged in the known stereo receiver, which circuit captures the noise components in the multiplex signal and which controls the threshold value and the hysteresis of the Schmitt trigger accordingly. A Schmitt trigger is herein understood to mean a switching amplifier with hysteresis, i.e. a circuit which may assume two switching states, dependent on the input signal, the transition from one state to the other being effected at a different level of the input signal than the transition in the opposite direction. In the known circuit the pilot signal detector comprises a bandpass filter tuned to the frequency of the pilot signal and succeeding a rectifier. The voltage at the rectifier output fluctuates to a proportionally small extent at small field strengths so that the constant switching of the Schmitt trigger can be substantially suppressed by causing the switching threshold and the hysteresis to follow.
In a pilot signal detector in which the output signal fluctuates to a consirderably stronger extent than in the known pilot signal detector, the known circuit principle cannot be used. For example, pilot signal detectors cooperating with a PLL circuit can supply output signals which may be both clearly above and clearly below the output level which is obtained in the case of undisturbed stereo reception.
It is an object of the invention to provide a circuit of the type desribed in the opening paragraph in which switching of the Schmitt trigger is substantially prevented, even at a strongly fluctuating output signal of the pilot signal detector.